The widespread and global deployment of fiber optic networks and systems mandates that fiber optic devices and components operate reliably over long periods of time. This mandate imposes stringent performance requirements on various fiber optic devices and components that are used in such networks and systems. In this respect, since fiber optic devices and components are expected to operate reliably for decades or more, prior to qualification for use, such components are typically subjected to an array of mechanical and environmental tests that are designed to measure their long term reliability.
Guarantees of long term performance become especially crucial in application where the cost of failure is very high (e.g., submarine applications.) One of these tests is a damp/heat soak test, where a fiber optic device or component is exposed to elevated temperature and humidity conditions (typically 85° C. and 85% relative humidity) for an extended period of time. Fiber optic couplers exposed to such conditions may exhibit a gradual drift in insertion loss. Eventually, this drift will cause a coupler to fail to meet its assigned performance specifications.
It is believed that the primary cause for the above-identified drift is water vapor or some component, constituent or by-product of water vapor diffusing into the exposed core glass of the coupler and changing the coupler's index of refraction.
In an attempt to prevent migration of moisture into the coupling region, it has been known to provide improved packaging for optic couplers, with the goal of eliminating exposure to external environment. For example, prior art approaches have included packaging fiber optic couplers and other fiber optic components inside a metal tubing and sealing the ends of the tubing with a polymeric material, such as a silicon-based material or epoxy. These types of packaging have not proved successful in preventing the aforementioned problem.
Other prior art approaches have focused on reducing the introduction of water vapor during the manufacturing process. These attempts include the use of heat sources, such as a solid state heaters alone, that introduce less hydrogen/water during fabrication of a coupler, than is introduced using an “open flame” heat source. However, these attempts have also failed. Such approaches are deficient because it has been discovered that the introduction of water and water related species during fabrication is not a major cause of long-term drift of optical properties under damp heat accelerated aging conditions. See Maack et. al, Confirmation of a Water Diffusion Model for Splitter Coupling Ration Drift Using Long Term Reliability Data. See also, Cryan et al., Long Term Splitting Ration Drifts in Singlemode Fused Fiber Optic Splitters, Proc. Nat. Fiber Opt. Eng. Conf. Jun. 18-22, 1995.